1. Field of the Invention
The invention relates to sol-gel processing methods for forming silica-based articles.
2. Discussion of the Related Art
Optical transmission fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element such as germanium, an inner cladding of high-purity silica glass optionally doped with a refractive index-lowering element such as fluorine, and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by forming an overcladding tube for the outer cladding, and separately forming a rod containing the core material and inner cladding material. The core/inner cladding are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD is discussed in co-assigned U.S. Pat. Nos. 4,217,027; 4,262,035; and 4,909,816. MCVD involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (known as the substrate tube) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch, is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated core rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into a solid, consolidated preform. It is possible to sinter a porous overcladding tube while collapsing it onto a core rod, as described in co-assigned U.S. Pat. No. 4,775,401.
Because the outer cladding of a fiber is distant from transmitted light, the overcladding glass generally does not have to meet the optical performance specifications to which the core and the inner cladding must conform. For this reason, efforts to both ease and speed manufacture of fiber preforms have focused on methods of making overcladding tubes. One area of such efforts is the use of a sol-gel casting process. Co-assigned U.S. Pat. No. 5,240,488 discloses a sol-gel process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In this process, a colloidal silica dispersion, e.g., fumed silica, is obtained having a pH of 2 to 4. To obtain adequate stability of the dispersion and prevent agglomeration, the pH is raised to a value of about 10 to about 14 by use of a base. Typically, a commercially-obtained dispersion is pre-stabilized at such a pH value by addition of a base such as tetramethylammonium hydroxide (TMAH). Introduction of the TMAH raises the pH value. Other quaternary ammonium hydroxides behave similarly. When the pH is so raised, the silica takes on a negative surface charge due to ionization of silanol groups present on the surface, in accordance with the following reaction:
xe2x80x94Sixe2x80x94OH+OHxe2x88x92xe2x86x92xe2x80x94Sixe2x80x94Oxe2x88x92+H2O.
The negative charge of the silica particles creates mutual repulsion, preventing substantial agglomeration and maintaining the stability of the dispersion. In this state, the zeta potential of the particles is at a negative value. (Zeta potential is the potential across the diffuse layer of ions surrounding a charged colloidal particle, and is typically measured from electrophoretic mobilitiesxe2x80x94the rate at which colloidal particles travel between charged electrodes placed in a solution. See, e.g., C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press, 242-243.)
At a later stage in the process, as discussed in Col. 15, lines 39-65 of the ""488 patent, a gelling agent such as methyl formate is added to reduce the pH. It is possible to use other esters, as well. The ester reacts to neutralize base, and the negative character of the silica particles is neutralized according to the following reaction:
xe2x80x94Sixe2x80x94Oxe2x88x92+H+xe2x86x92xe2x80x94Sixe2x80x94OH.
A sufficient amount of the ester must be introduced to neutralize the silica to a degree where gelation is induced. (Gelation, as used herein, indicates that the colloidal silica particles have formed a three-dimensional network with some interstitial liquid, such that the dispersion becomes essentially non-flowing, e.g., exhibiting solid-like behavior, at room temperature.) Subsequent to gelation, the sol-gel body is typically released from its mold, and placed in an oven for drying, heat treatment, and sintering, as reflected in the Table at Cols. 11-12 of the ""488 patent.
As the capabilities of such sol-gel techniques have improved and expanded, there has been an increasing desire to utilize sol-gel methods for more aspects of the optical fiber fabrication process as well as to apply sol-gel to other optical applications. Processes for doing so would be desirable.
The invention provides a sol-gel process for fabricating bulk, germanium-doped silica bodies useful for a variety of applications, including core rods, substrate tubes, immediate overcladding, pumped fiber lasers, and planar waveguides. The process involves the steps of providing a dispersion of silica particles in an aqueous quaternary ammonium germanate solution typically tetramethylammonium germanate, gelling the dispersion to provide a gel body, and drying, heat treating, and sintering the body to provide the germanium-doped silica glass.
The step of providing the dispersion typically involves obtaining hexagonal germanium dioxide powder, mixing the germanium dioxide powder with an aqueous solution of quaternary ammonium hydroxide to provide the quaternary ammonium germanate solution, and mixing the silica with the quaternary ammonium germanate solution. (As used herein, quaternary ammonium germanate solution indicates the result of mixing germanium oxide with an aqueous quaternary ammonium hydroxide solution.) The step of gelling the dispersion typically involves adding a second base to the dispersionxe2x80x94which raises the isoelectric point, aging the dispersion, and then adding a conventional gelling agent to reduce the pH to induce gelation. (The isoelectric point is the point on the pH scale where the zeta potential is zero, as discussed in C. J. Brinker and G. W. Scherer, Sol-Gel Science, supra. Gelling agent indicates a material capable of inducing gelation of a silica dispersion.)
The process has numerous benefits. For example, the process enables fabrication of an entire fiber preform by sol-gel techniques. In fact, formation of a Ge-doped core rod by the sol-gel technique is expected to be easier than conventional techniques such as MCVD. The sol-gel bodies also retain more of the germanium than such conventional techniques. In fact, the sol-gel bodies of the invention tend to retain at least 80% of the germanium introduced into the gel body (the remainder lost during, e.g., heat treatment and sintering). This makes the process more efficient and also provides a way to readily provide the high doping levels required by some applications, e.g., multimode fibers used for LAN/WAN applications. It is also possible according to the invention to fabricate germanium-doped silica bodies in a variety of shapes and sizes, including films or even powders, simply by casting the gel in an appropriate configuration.
In addition, it was found that commercially available germanium dioxide, which is marketed as being hexagonal, actually contains a small amount of tetragonal germanium dioxide. This tetragonal germanium dioxide is insoluble in the quaternary ammonium hydroxide solution and is thus undesirable in the process of the invention. Applicants discovered a process, however, for forming essentially 100% hexagonal germanium dioxide, by a technique that is actually much simpler than the conventional method. Specifically, the conventional method involves hydrolysis of germanium tetrachloride with aqueous ammonia. (See, e.g., F. Glocking, The Chemistry of Germanium, Academic Press, 35 (1969).) By contrast, according to an aspect of the invention, the hexagonal germanium dioxide is provided by a process involving steps of adding germanium tetrachloride to a large excess of deionized water, advantageously under vigorous stirring, such that germanium dioxide precipitates out. (Large excess indicates a volume ratio of water to germanium tetrachloride of at least 3:1, and typically 3:1 to 10:1.) The precipitate is then washed and dried to provide essentially 100% hexagonal polycrystalline germanium dioxide, i.e., x-ray diffraction indicates the presence of only the hexagonal form. (It is possible, though unlikely, that some amorphous germanium dioxide is present, since the amorphous material is not detectable by x-ray diffraction and is soluble in the quaternary ammonium hydroxide solution.) This formation process facilitates successful fabrication of the germanium-doped silica bodies of the invention.